A drawing apparatuses which is exemplified in a printer and can make drawings on media such as paper, employs an inkjet print head which ejects ink droplets to make drawings. A thermal inkjet printing is one of inkjet printing technologies, such that an electric current is sent to a heating resistor to heat up ink and ink droplets are ejected from a nozzle by pressure of vapor bubbles generated on the heating resistor. As a kind of inkjet print head employing the thermal inkjet printing, there is known an inkjet print head manufactured by combining a substrate on which a heating resistor and a drive circuit are formed and a substrate on which nozzles are formed. From a point of variation in manufacturing accuracy of the substrates and a point of positioning accuracy of the substrates in the process of combining the substrates, the following inkjet print head has been proposed and employed. For example, as described in Japanese Unexamined Patent Application Publication (JP-A) No. H11-99649, the inkjet print head has the following structure: there is provided a silicon substrate on which a heating resistor is formed, and an orifice plate which has ink ejection ports is put on the silicon substrate to be integrated in one body.
From a viewpoint about drawing processing, print heads can be categorized into those employing a scan printing method and those employing a single-path printing method. In the scan printing, a print head performs plural scans (reciprocates) in a direction perpendicular to the conveyance direction of print media. In the single-path printing, a print head remains in a fixed position and completes image printing in a single path on the print media under conveyance. Among printers employing such the technologies, line printers employing the single-path printing method are desired because of their high-speed capability. In the single-path printing method, a print head or print heads need to be arranged in proportion to the size of print media. Therefore, in a line printer for printing on large-sized print media such as an outdoor advertisement, a large number of print heads are arrayed. Especially from a point of resolution, a printer needs a large number of microscopic print heads.
When inkjet print heads using a silicon substrate are employed in such printers, the following problem arises. Since it is generally difficult to create a large-sized silicon substrate and the number of inkjet print heads which can be formed on the substrate is limited, such printers need to use plural inkjet print heads combined together. When a printer employing combined inkjet print heads performs printing, a printed image can have a gap, depending on the processing accuracy and combining accuracy of the inkjet print heads, at a position corresponding to the position where the inkjet print heads are combined together. The gap becomes more conspicuous in high-quality printing. Especially, the single-path printing method is a method that a printer head or printer heads complete image printing in a single path and cannot employ a way to correct the gap during plural scans, which is a problem.
On the other hand, a glass substrate, which is used for technologies including a liquid crystal display device, is manufactured in much larger size in comparison with a silicon substrate, and can be manufactured at low cost. Therefore, there has been proposed a method of manufacturing an inkjet print head on a glass substrate. For example, JP-A No. 2000-289204 discloses an inkjet print head integrated with a driver, wherein the inkjet print head is formed on a glass substrate and includes a heating resistor.
In a print head using the thermal inkjet printing, ink has a temperature as high as several hundred degrees Celsius around a heating resistor. For achieving high-speed printing, a print head needs to repeatedly eject ink droplets at high speed. However, if heat of the print head is accumulated to make the temperature of the print head excessively high, such the condition can make the print head impossible to eject a proper amount of ink and can make a large number of inferior printings, which is a problem. To solve of the problem, a print head needs to conduct and radiate the heat generated in the heating resistor in short time. JP-A No. 2001-191529 discloses the following print head. The print head includes a substrate and a metal heat sink layer. The metal heat sink layer adjoins the substrate and has a plan view shape substantially the same as and congruent with the plan view shape of the substrate. The metal heat sink layer efficiently removes heat generated by resistors or other energy dissipating elements of the print head. Further, JP-A No. 2003-170597 discloses the following inkjet print head. The inkjet print head includes a substrate having a heat conductivity being equal to or less than 15 W/m/K, a heat-conductive layer being equal to or more than 10 μm in thickness and being put on the substrate, a heat insulation layer put on the heat-conductive layer, and a heater put on the heat insulation layer. In the inkjet print head, the heat-conductive layer controls an increase of the temperature around the heater put on the substrate having a low heat conductivity.
Further, JP-A No. 2002-316419 discloses the following inkjet print head which can radiate heat outside efficiently. The inkjet print head includes a glass substrate having top surface and bottom surface on each of which a metal film is formed. On the metal film on the top surface, the following components are layered in order: an insulating film, a heating resistor film, electrodes of individual wirings, a common electrode, barriers and an orifice plate including ink ejection openings. The glass substrate is cut to make an ink supply channel and ink supply openings. In the glass substrate, a through hole is further made and a metal film is formed on the inner wall of the through hall to connect the metal films formed on the both surfaces of the substrate together by thermal coupling.
Regarding a method of manufacturing an inkjet print head, JP No. 2003-36956 discloses the following method of manufacturing a heating resistor. The heating resistor includes an alumina substrate having an excellent heat conductivity and a heat storage layer put on the alumina substrate. With the method of manufacturing the heating resistor, a heat radiation layer and the heat storage layer are formed efficiently with having the well-balanced heat radiation property and heat storage property.
However, the above-described conventional arts have several problems.
A first problem is that they do not show promise of an excellent efficiency of heat conduction. In order to lower the temperature of an object having a quantity of heat, it is important to reduce the quantity of heat of the object by using heat conduction. A heat conductance, in other words, the ease with which a particular material conducts heat can be expressed by the following formula (1). The parameters of the formula are important for an efficient conduction of heat in components such as a heating resistor.“Heat Conductance”=“Heat Conductivity of the Material”×“Cross-Sectional Area for Heat Conduction”/“Length of the Material”  (1)
In the inkjet print head disclosed in JP-A No. 2000-289204, a heating layer is covered with a glass substrate, a silicon oxide film and a silicon oxynitride film, which are formed of materials with low heat conductivity. Therefore, the efficiency of the conduction of heat coming from the heating layer deteriorates.
Further, in inkjet print heads disclosed in FIG. 6 and FIG. 7 of JP-A No. 2003-170597, a heat conductive layer for releasing heat generated in the heating resistor, is not arranged in an area of a drive circuit. As described above, heat conduction between solid bodies is proportional to a contact area of layers or films where heat passes. As an inkjet print head is much more downsized, the ratio of an area of the drive circuit in an inkjet print head becomes greater, which reduces an area to contribute to the heat conduction and makes the heat conduction in the inkjet print head difficult.
A second problem is that thin-film transistors (hereinafter, referred to as a TFTs) having sufficient performance are hardly obtained in the above technologies. In a thermal inkjet print head, an electric current is sent to a heating resistor in a short cycle of time (for example, a cycle of the order of magnitude of microseconds) in a printing process. Therefore, a transistor to be connected to the heating resistor needs to have high performance, for example, high mobility. Further, in an inkjet print head integrated with a drive circuit, TFTs as the components of the drive circuit also need to have high performance.
In the print head disclosed in JP-A No. 2001-191529, there is a metal heat sink layer for absorbing heat from a resistor and radiating excess heat therefrom, extending all over the substrate. As a process being used for forming high-performance TFTs on a glass substrate, a process of crystallizing silicon by laser annealing, using an excimer laser is well known. In the process, amorphous silicon as a precursor is deposited at a position apart from a glass substrate in order to prevent the amorphous silicon from being contaminated by impurities coming from the glass substrate. Therefore, in the print head disclosed in JP-A No. 2001-191529 wherein a metal layer is put immediately above the substrate, the a metal layer is located under the amorphous silicon layer. However, on crystalizing silicon by laser annealing, if the metal layer is located under the amorphous silicon layer, heat for melting the amorphous silicon easily leaks out to the metal layer and the amorphous silicon does not reach a sufficient temperature. It makes difficult to prepare crystals having large crystal-grain sizes and to achieve high-performance TFTs having high mobility. This issue can arise similarly in the inkjet print head disclosed in FIG. 5 of JP-A No. 2003-170597, which includes a heat conductive layer put between an active layer (a layer of polycrystalline silicon) and a substrate, and the inkjet print head disclosed in JP-A No. 2002-316419, which includes a metal film formed on the top surface of a substrate. The issue can arise similarly further in the technique to form a heat storage layer and a heating resistor film on an alumina substrate, as disclosed in JP-A No. 2003-36956. That is, when TFTs are being formed on the alumina substrate having an excellent heat conductivity, heat given for melting the amorphous silicon in a process of crystallizing the amorphous silicon by laser annealing easily leaks out into the alumina substrate in spite of existence of the heat storage layer, which makes difficult to achieve high-performance TFTs.
Further, in an inkjet print head having a heat conductive layer being patterned rather than being formed to cover all over the substrate, as disclosed in FIG. 6 of JP-A No. 2003-170597, the process of crystallizing amorphous silicon makes a polycrystalline silicone layer having crystals whose crystal-grain sizes are uneven greatly in the plane of the substrate. Such a condition can cause the problem that the etching rate of dry etching or wet etching varies depending on a position on the substrate and fine silicon residues are created, which produces defects of the inkjet print head. This problem arises because of the following reason. The temperature of a part where the heat conductive layer exists does not rise but the temperature of a part where the heat conductive layer does not exist rises. Depending on existence or nonexistence of the heat conductive layer, there can be created a silicon layer including crystals with uneven grain sizes. A silicon crystal of a small grain size has the great grain boundary and the etching rate for the crystal becomes greater in comparison with a silicon crystal of a larger grain size, which causes this problem. This phenomenon can also be observed in other materials such as silicon carbide (SiC).
The present invention seeks to solve the problems.